Electromagnetic focus coil for cathode ray tube



Sept. 18, 1956 s DE L 2,763,805

ELECTROMAGNETIC FOCUS COIL FOR CATHODE RAY TUBE Filed June 29, 1954 2Sheets-Sheet 1 IN V EN TOR.

Sept. 18, 1956 s. 1.. BENDELL 2,763,805

ELECTROMAGNETIC FOCUS COIL FOR CATHODE RAY TUBE Filed June 29, 1954 2Sheets-Sheet 2 JVE/YHL 01/7707 IN V EN TOR. Jan 5y Z. fli/yafl UnitedStates Patent O ELECTROMAGNETIC FOCUS COIL FOR CATHODE RAY TUBE SidneyL. Bendell, Haddon Heights, N. 3,, assignor to Radio Corporation ofAmerica, a corporation of Delaware Application June 29, 1954, Serial No.440,039

Claims. (Cl. 313-84) The present invention relates to new and improvedelectromagnetic focus coils of the type employed in focussing anelectron beam within a cathode ray tube such, for example, as a cathoderay television camera tube.

As is well known, conventional cathode ray tubes include a target of atype depending upon the function of the tube and an electron gun forforming an electron beam which is caused to impinge upon the target byvirtue of a potential difference between the gun and the target. In thetelevision field such devices are employed for a variety of uses,including the derivation of electrical signals representative of asubject and also as image reproducing devices which operate to form alight image reconstructed from such signals. It has been found necessaryto provide some form of apparatus for focussing the electron beam insuch tubes to provide a fine spo on the target, in order for acceptableimage resolution to be afforded. In view of the necessity for focussingmeans, various proposals have been made in the prior art which fallprimarily into two main classes, namely, electrostatic and magneticarrangements. The latter class includes permanent magnet focussingdevices and electromagnetic focussing devices both of which have incommon the fact that they provide a magnetic field whose flux lines aresubstantially parallel to the nominal path of the beam. That is to say,the magnetic field is axial with respect to the tube.

Insofar as the electromagnetic focussing coil art is concerned, oneproblem has been that of forming a focus coil of the proper axial lengthand with the required number of conductor turns needed for focussing ofthe electrons in order to meet the requirement that the field besubstantially uniform throughout its axial length. The problem inquestion has become even more aggravated in the case of multiple cathoderay tube arrangements wherein several cathode ray tubes are caused toscan a common object, as in a color television camera, for example.

It is, therefore, a primary object of the present invention to provide anovel electromagnetic focussing coil for use in conjunction with acathode ray tube.

Among the various methods of winding focussing coils employed in thepast, characterized in that a conductor is wound about a cylindricalcoil form, are the so-called randon winding and layer winding methods.Random wound coils are normally made by hand and have the disadvantagesof being costly to produce and of lacking uniformity with respect toother coils. Such a method of winding coils for focussing is, therefore,undesirable where it is necessary that a plurality of focussing coils beidentical so that the cathode ray tubes with which they are associatedmay be operated in unison with respect to a common subject. Layerwinding of focussing coils with layer-winding machines has also beenfound to be a source of difficulty, particularly where it is attemptedto layer-wind a coil without the interposition of paper insulatingsheets between successive layers. That is, the

ice

layers tend to become jumbled or non-uniform, so that the machineoperator must thereafter exercise his judgment as to how best tocompensate for the non-uniformity. It is readily apparent that such amethod depends to a large extent upon the skill and judgment of theoperator.

Hence it is a further object of the present invention to provide a novelelectromagnetic focussing coil which is simple to form with readilyavailable commercial winding apparatus and which does not give rise tosituations requiring the exercise of judgment on the part of the personoperating the winding apparatus.

In general, the present invention is characterized in that it providesan electromagnetic focussing coil comprising" a cylindrical coil form ofsuitable dimension for surrounding the neck of the cathode ray tube withwhich the coilis to be associated and a conductor wound about the formas a so-called universal winding. That is to say, the conductor is woundabout the cylindrical form helically and with such a large pitch thatsuccessive convolutions of the conductor are spaced axially from eachother, the conductor then being wound back over itself with the reversepitch. The present invention affords the advantages that coils producedin accordance with its principles are self-supporting (i. e. do notrequire end supports), are extremely precise in their turns distribution and may be produced by mass production methods with extremeuniformity between coils.

Additional objects and advantages of the present invention will becomeapparent to persons skilled in the art from a study of the followingdetailed description of the accompanying drawings, in which:

Fig. l is an elevational view of an electromagnetic focus coil of theinvention;

Fig. 2 illustrates diagrammatically, and in a simplified manner,apparatus by means of which the coil of Fig. 1

may be wound; and

Fig. 3 is a schematic diagram of a tricolor television cameraarrangement in which coils of the present invention may beadvantageously employed.

Referring to Fig. 1 of the drawing, it will be seen that theelectromagnetic focus coil of the present invention comprises acylindrical core or coil form 10, about which there is wound a wireconductor 12 in the form of a universal winding. That is to say, as maybe seen from the drawing, the conductor is so wound about the form 10 asto have a substantially larger pitch than that which would be requiredfor successive convolutions of the coil to be contiguous. In fact, thepitch is of such value that successive helical turns of the conductorare spaced axially from each other by an appreciable dis tance. Whilethe term universal winding is well known to those engaged in the textilearts, wherein thread, twine and the like are wound in that manner, themethod by which the universally wound focus coil of Fig. 1 isaccomplished may be better understood by those skilled in theelectronics art from the showing of Fig. 2.

In Fig. 2 there is illustrated, by way of a simplified isometric view awell-known arrangement for producing universal windings of cord or thelike and which has been found quite effective in winding the cathode raytube focus coils of the present invention. Specifically, the coil form10 is supported on a shaft 14 driven through suitable gearing 16 and 18by a motor shaft 20. The rotation of the coil form 10 is, by way ofillustration, in the direction of the arrow 22. The higher conductor 12is supplied from a reel 24 supported in suitable relationship withrespect to the rotating coil form 10 and the wire feeding apparatusindicated in its entirety by reference numeral 26. In general, theconductor 12 is, after being connected at one end 12 to the coil form asthrough the frictional engagement by the sides of a slot 28 in the endsupport 30, is fed to the coil form 10 by means of a laterallyreciprocating arm 32 which carries a V-grooved wire feeder roll 34 atits extremity adjacent the coil form 10. The arm 32 is caused toreciprocate in the directions indicated by the arrow 36 through asuitable mechanical arrangement synchronized with the form rotatingmechanism as by means of the idler gear 38 enmeshed with the motor gear18 and a cam gear 40. The cam gear 40 drives a shaft 42 through theagency of a gainer mechanism 44 which is in the nature of an escapementmechanism whose function will become more fully apparent hereinafter.Rigidly mounted on the shaft 42 is a nutating cam wheel 46 which isprovided with a cam groove 48 in its periphery. The end of thewire-feeding arm 32 remote from the wheel 34 is provided with a lowfriction cam follower 59 which rides in the groove 48. Thus, rotation ofthe shaft 42 causes nutating movement of the cam wheel 43 in such manneras to cause reciprocation in the horizontal direction (arrows 36) of thewire feeding arm 32 back and forth per revolution of the cam. The arm 32is illustrated diagrammatically as being supported and guided in itsreciproeating movement by means of a slide member 52 which rides in abearing slot 54 of a support member 56.

The gear ratio of the train employed is so chosen that the coil formexecutes a plurality of revolutions for each traverse of the wirefeeding arm 32 between one of its extreme positions and its otherextreme positions. That is to say, the shape of the cam 46 and its speedof rotation is such that, at the beginning of operation, thewire-feeding roller 34 is at the end of the coil form 10 adjacent themember 30 and, several revolutions of the coil form later (dependingupon the gear ratio 16, 18, 38 and 44), the roller 34 has traversed thelength of the coil form until it is at the end thereof remote from thearrangement 30. In Fig. 2, the apparatus is shown in its position afterseveral revolutions of the form 10 in the direction of the arrow 22 whenthe arm 32 is approaching the extremity of its lateral excursion to theright. Such relative movement of the coil form 10 and the wire feeding:arm 32 causes the conductor 12 to be laid down helically, as shown, andwith successive convolutions thereof spaced apart axially the distanced. In the terminology of the textile art in which apparatus of the typeshown in Fig. 2 is commonly employed for winding thread or twine onto abobbin, the distance d is termed the cross-over per turn. Depending,therefore, upon the value of cross-over per turn, the point at which theconductor 12 reaches the end of the coil form and begins its helicaltravel back toward the starting point will vary radially of the coilform. At the time that the wire-feeder arm reaches its extreme righthandposition in Fig. 2, the cam groove 48 will cause the arm 32 abruptly tocom mence lateral movement to the left, so that the conductor 12 willthen be wound in the same manner, but with the reverse pitch, over thefirst group of turns.

The gainer mechanism 44, which forms a conventional adjunct tocommercial thread winding machines such as those sold commercially underthe style Universal Winding Machine by the Universal Winding Company ofCranston, Rhode Island, comprises means for providing a minute shift ofthe positioning of the cam shaft 42 to compensate for the diameter ofthe filament being wound, so that successive groups of convolutions lieadjacent each other rather than directly on top of each other. Since thespecific apparatus for forming the focussing coils of Fig. 1 does notconstitute a part of the present invention, additional detaileddescription is unnecessary here. It will be understood, however, thatthe gainer effectively adds an increment to the displacement of the camafter each complete rotation thereof, whereby to increase the wire pitcha slight amount.

In order that the above description of the typical universal windingmeans may be better correlated with a specific coil, it will be seen inFig. 1 that the conductor is neatly and precisely arranged on the form10 with successive cross-over points spaced from each other. Thedimension cross-over per turn is indicated by the distance d in Fig. 1and the coil in that figure is properly termed a 3-cross-over-per-turnuniversal winding.

One environment in which focussing coils in accordance with the presentinvention offer great utility is that illustrated diagrammatically inFig. 3, wherein there is shown a conventional 3-Vidicon television colorcamera of conventional form. Each of the three Vidicons 6t), 62 and 64is of the type described in detail in an article entitled The Vidiconphoto conductive camera tube by Weimer, Forgue and Goodrich,Electronics, May 1950. The Vidicon, as illustrated in a greatlysimplified manner in the drawing comprises an electron gun 64 located atone end of the evacuated cylindrical glass envelope 66, which produces abeam of electrons 68 for impingement upon a pho-toconductive targetelectrode 70 located at the other end of the envelope 66. The electronbeam 68 is caused to scan a conventional, rectangular raster by means ofsuitable sawtooth currents of field and line frequency flowing throughthe associated vertical and horizontal deflection coils 72 and 74,respectively. In order that the electrons of the beam 68 may be focussedto a fine spot at the target 70, whereby acceptable resolution of theimage may be realized, each of the Vidicons 6t), 62 and 64 is providedwith an electromagnetic focus coil 76. Light from a subject in naturalcolor (indicated by the X) is focussed via a suitable lens arrangement78 onto a color selective light splitting arrangement 80 which may, forexample, comprise the well-known di chroic reflectors 82 and 34 of thetype described in detail in U. S. Patents 2,379,790 and 2,312,492,granted July 3, 1945, and December 10, 1946, respectively, to G. L.Dimmick. The optical arrangement of the dichroics 8t 82 and 84 isexplained in U. S. Patent 2,672,072, granted March 16, 1954, to L. T.Sachtleben et al. for Color Television Optical System.

Briefly, the dichroic reflector 82 reflects red light from the subjectto the Vidicon 62 and passes the blue and green light. The dichroic 84reflects the blue light to the Vidicon 64 and permits the green light topass to the Vidicon 60. As pointed out in the above-cited Sachtleben etal. patent, the lengths of the paths from the subject to the threecameras are equal to each other, in order that the size of the imagefocussed onto the tubes is the same for the three pickup devices. Sinceit is necessary, for proper registration of the three component colorimages, that the images be scanned in unison by the three camera tubesand with the same degree of preciseness, the universally wound focussingcoils of the present invention are well adapted for such use. By way ofexample, the focus coils 76 are illustrated as being energized from thesame source of focus voltage 86, in a wellknown manner. This commonenergization may be from a constant current source feeding the severalfocus coils in series, as is conventionally done.

While, as will be appreciated, the specific dimensions andspecifications for electromagnetic focussing coils wound in accordancewith the invention will vary, depending upon the use to which the coilsare to be put, there follows by way of example a description of onefocus coil, designed for use with a Vidicon television camera tube,which has been found to be extremely satisfactory in use:

Outer diameter of coil form=2.843 Outer diameter of focus coil=3.2l8"Axial length of focus coil=4 A cross-over per turn Throw of cam 46 (Fig.2)=4" Ratio of gears 16 to 40:33/109 Number of turns=8500 Wire type: No.33 single silk enamel wire Outer diameter of wire=0.0092

The specific Vidicon type with which the above-described coil wasdesigned for use is the RCA 6326 Vidicon.

From the foregoing, those skilled in the art to which the presentinvention apertains will recognize that the novel universally woundelectromagnetic focus coils of the invention are simple to produce withmass production methods, yet possess a degree of preciseness unequalledby coils constructed according to the usual prior art methods.Additionally, the fact that the coils of the invention may be wound tomeet rigid specifications renders them extremely suitable for suchenvironments in which several coils are required to have exactly thesame characteristics and to produce identical focussing fields. Thepreciseness of the coils wound in accord ance with the present inventionis, as stated, of particular importance by reason of the fact thatvariations in the axial magnetic focussing field which would be producedwith non-uniformly wound coils are known to cause changes in thegeometry of the scanned raster. Thus, where a plurality of rasters mustbe identical to each other, it is essential that the flux patternsproduced by the focussing coils be identical, which requirement isreadily met by the coils of the present invention.

While the invention has been described in accordance with a specificform in which the electromagnetic focus is in the form of a singleuniversal winding, it should be borne in mind that, where desired as forpurposes of facilitating manufacture or for electrical reasons, thefocus coil may be made of a plurality of separate universally woundcoils arranged coaxially alongside each other.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent:

1. An electromagnetic focus coil adapted to surround at least a portionof a cathode ray tube of the type having a target and means fordirecting a beam of electrons along a path toward such target, said coilbeing adapted to provide an axial magnetic field and comprising a coilform of substantially cylindrical shape and a conductor wound thereon asa universal winding of a plurality of crossovers per turn.

2. An electromagnetic focus coil adapted to surround a portion of acathode ray tube, said coil comprising an elongated, cylindrical coreand a conductor wound there- 6 on helically from one end thereof to theother and back, and of a pitch substantially greater than that necessaryfor successive helical turns to be contiguous, latter turns of saidconductor being wound over prior turns and with reverse pitch.

3. An electromagnetic focus coil for a cathode ray tube, said coilcomprising: an elongated cylindrical form and a conductor wound thereonhelically from a first end thereof to the second end thereof and withsuch certain pitch that successive convolutions are spaced from eachother axially of said form, said conductor being wound back over itselffrom said second end to said first end with a pitch substantially equalto such certain pitch but of the reverse direction.

4. An electromagnetic focus coil for use in conjunction with a cathoderay tube of the type having a generally cylindrical portion, a target,and means for directing a beam of electrons along a path within suchgenerally cylindrical portion toward such target, said coil beingadapted to surround at least a portion of such path and comprising asubstantially cylindrical coil form and a conductor wound thereon as auniversal winding of a plurality of layers and having a plurality ofcrossovers per turn.

5. An electromagnetic focus coil for use in conjunction with a cathoderay tube of the type having a generally cylindrical portion, a target,and means for directing a beam of electrons along a path within suchgenerally cylindrical portion toward such target, said coil beingadapted to surround at least a portion of such path and comprising asubstantially cylindrical coil form and a conductor wound thereon as auniversal winding of a plurality of layers and having a number ofcrossovers per turn greater than two.

References Cited in the file of this patent UNITED STATES PATENTS2,071,516 Farnsworth Feb. 23, 1937 2,217,409 Hepp Oct. 8, 1940 2,243,893Blurnlein June 3, 1941 2,296,355 Levin Sept. 22, 1942 2,298,673 Brown eta1. Oct. 13, 1942 2,420,156 Van Suchtelen May 6, 1947 2,550,592 PearceApr. 24, 1951

